growth rate is a rather unusual one.* We do not know enough about the facts to examine this element in more than a suggestive way. The discussion that follows suggests that the sib-number effect, the sib-spacing effect, and the parental-mortality effect are all detrimental to the quality of the work force, but we do not know to what degree. The replacement effect, considered at the end of the paper, may operate in the opposite direction from the other effects considered, in some cases. THE NURTURE-NATURE BORDERLINE: INTELLIGENCE AND FAMILY SIZE An interesting qualitative aspect of the problem involves the imprecise borderline between nurture and genetic inheritance. A large (although controversial) literature has accumulated which suggests that, on the average, children that come from families with relatively few siblings or with no siblings do disproportionately better at intellectual and related pursuits than those with many siblings (10). Also birth order is in some degree connected with intellectual achievement—on the average the higher the birth order the greater the achievement. It has not been determined whether any of this contains a genetic component. At present we know little about the relation between population quality, entrepreneurial capacities, innovating capacities, and contact with siblings. There are some data that suggest that an unusual proportion of those who have considerable intellectual achievements to their credit were either only children or from families in which there was a relatively large age gap between siblings (11). It seems plausible that the ability to think abstractly would be developed earlier or would on the whole be greater if children learned the concomitant verbal skills either from adults or from siblings considerably older than themselves (12). Although it is difficult in fact to separate the level of intelligence from acquired skills, there is evidence to suggest that a child's intelligence level can actually be raised by a culturally nurturant upbringing or by training (13), or by the kinds of environmental stimuli available in an urban setting (14,15); and that there is a connection between family size and intellectual capacity. Intellectual capacity with the attendant ability to manipulate abstractions that typifies educated intelligence is unquestionably important to economic development. It is evident in the contribution of professionalized skills to the economy; i.e., in the work of engineers, lawyers, doctors, architects, and teachers at various levels. It seems likely that acquired intellectual capacities are also related to managerial skills. It would appear then that the smaller the rate of population growth and the smaller the family size, the greater the extent to which these skills could be developed. *To some degree this idea is found in the concept of "embodied technical change."